Summary A disruption in blood flow due to age, disease or injury has been linked to imbalances in bone homeostasis and fracture healing processes. However, little is known about the interplay between blood flow within skeletal capillaries, the heterogeneity of vascular endothelium, and the osteogenic potential of residing osteoprogenitor cells. Current knowledge about skeletal capillaries has been primarily provided by the use of rodent models. Recently, two distinct capillary sub-types coined as type H and type L were discovered in long bones of mice. A decline in type H capillaries has been also linked to a decline in osteogenesis and overall bone mass in both aged mice and in osteoporotic human subjects, however the underlying reasons remain poorly understood. Taking inspiration from recent work in rodent models, we will develop a new in vitro human skeletal microvascular model to study the role of microvascular physiology in regulating osteogenesis. Endothelial (ECs) and osteoprogenitors (OPs) will be derived from human induced pluripotent stem cells (hiPSCs), and incorporated into biochips created through a new Hybrid Laser Printing (HLP) technology. Three specific aims are proposed. Aim 1 will use HLP to print hydrogel biochips with embedded channel networks that resemble the serial arrangement of type H and L capillary morphologies. Flow velocities, shear stresses and diffusion properties of embedded channels will be characterized. Aim 2 will endothelialize the biochips with hiPSC-ECs, and endothelial barrier function, regional hypoxia, vessel sprouting, and expression of flow-regulated genes will be assessed in each type of vessel. Aim 3 will characterize the role of microvascular flow on osteogenic potential using hiPSC- OPs. In summary, this project will develop a human skeletal microvasculature model for studying the structure- property relationships between vessel morphology, its influence on endothelial heterogeneity and angiogenic function, and corresponding physiological consequences on osteogenic potential. In the long-term, this model can be broadly applied to screen clinically relevant therapeutics that target capillary beds of the skeletal system as well as other organ types.